<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <style>
        .aa{
            background: aqua;
        }
    </style>
</head>
<body>
 <table border="1">
     <tr >
         <td colspan="3" width="300px" class="aa">a</td>
     </tr>
     <tr>
         <td>4</td>
         <td>5</td>
         <td>6</td>
     </tr>
     <tr>
         <td>7</td>
         <td>8</td>
         <td>9</td>
     </tr>
 </table>
</body>
</html>